Manual intravenous systems typically control flow by means of roller clamp devices. Such devices have the virtue of simplicity, but are sometimes difficult to adjust and are not amenable to insertion in a servo assembly for automatic control of flow rate. Moreover, such manual devices may not permit control of flow rate with sufficient precision. On the other hand, automatic flow control devices commonly require insertion directly in the flow path of the fluid, and in instances when indirect mechanical control of flow is achieved without insertion into the flow path, there are problems in achieving a fail-safe mode of operation and in permitting disengagement of the control device in such a way that flow continues at a rate last adjusted by the device.